This invention relates generally to a high pressure liquid chromatography (HPLC) solvent delivering system and, more particularly, to an HPLC system capable of delivering constant flow of solvent mass.
HPLC solvent delivery systems have the primary objective of delivering constant flow of solvent mass through an HPLC column. Many HPLC systems utilize a single piston plunger pump that runs in a "open loop" mode with no measurements of actual solvent conditions. Under ideal conditions, a single piston plunger pump can deliver relatively constant flow over the long term, but produces individual cycles of fluctuating flow. The fluctuations in solvent flow usually result from variations in the time periods required during a given pump cycle for compression of liquid solvent prior to delivery and for full decompression of the pumping chamber after a delivery period. Factors affecting the lengths of the compression and decompression periods include the inadvertent presence in the pumping chamber of gasses, system leaks and changes in solvent compressibility. The presence of gasses in the pumping chamber may completely prevent solvent delivery, but in all cases produces an error in delivery volume that is undetectable by the pump.
Because of the difficulty of controlling mass flow directly, existing HPLC systems typically rely on precision manufacture of volumetric chambers, precision control of reciprocating displacement plungers, and carefully controlled conditioning of the solvents being pumped. Solvent conditioning reduces the possibility of developing gasses in a pumping chamber while precision manufacture of chambers and position control of reciprocating plungers enhances the predictability of pump delivery volume. For example, attempts are often made to minimize a pump's dead volume and thereby reduce flow delivery errors that occur during a pump's decompression period. However, dead volume can not be fully eliminated with practical methods and in any case there are numerous advantages to the use of a variable displacement pump which inherently exhibits dead volume.
An early improvement in HPLC systems employed a pair of interconnected piston pumps having piston plungers that operated with overlapping motions to thereby substantially smooth plunger cycle variations. However, the overlapped plunger motion of such systems can only be optimized for single solvent compressibility, and the systems retain a requirement for highly controlled conditioning of solvent to prevent the appearance of gasses.
A more recent HPLC system claiming improved solvent flow characteristics is disclosed in U.S. Pat. No. 4,919,595. That system included in a pumping chamber a pressure transducer to provide pressure information used to determine the loss of solvent flow that occurs during refill and compression periods of a pump cycle. The measured volume of deficit flow then is introduced to the system by increasing the velocity of the piston plunger during that period of a delivery stroke immediately following a refill stroke. Although compensating for deficit flow, the disclosed system doesn't consider the presence of gas in the pump and consequently requires careful conditioning of solvent because flow compensation is limited to the volume of surplus flow in the remaining stroke of the piston plunger. In an extreme case of a large gas bubble being ingested by the pump, an entire cycle can be completed compressing only gas and resulting in no deficit flow compensation.
Many HPLC systems employing single piston plunger pumps utilize a pulse damper to smooth the flow of solvent to a column. The damper accumulates solvent during the pump's delivery stroke and delivers solvent during the pump's refill stroke. Although improving the long term flow characteristics of the system, the solvent flow versus time output characteristic of a damper is a saw-tooth function in which solvent flow cyclically varies above and below average flow for the system. The cyclical periods of non-average flow are detrimental to chromatographic analysis.
The object of this invention, therefore, is to provide an improved HPLC system that alleviates the above described problems.